This invention relates to optical systems, and more particularly to curved mirror optical systems including two mirror magnifying systems and two and three mirror periscope systems.
Multiple mirror optical systems usually utilize two or more plane surface mirrors. In these systems, lenses which are rotationally symmetric are often mounted along the axis of symmetry of the system. Standard ray tracing para-axial lens mathematics is used to achieve the optical engineering goals. Typical of such systems are the common binocular and periscope systems. The use of curved mirrors in optical systems has been much more limited and have been characterized by excessive structural and optical complications, and correspondingly high cost of manufacture, while producing images of reduced quality.
A matched pair of curved mirrors are used in various telescope, microscope and projection systems. Typical of such a system is the Cassegrain telescope with its various modifications, including the Ritchey-Chertein telescope using two hyperboloid mirrors (see U.S. Pat. No. 3,752,559). In these telescope systems the light from objects being viewed is first reflected by a large primary mirror onto a smaller secondary mirror which is positioned at the center of the field of view of the primary mirror. The optical axis is identical with the path of a light ray from the eye of the observer to an object at the center of the field of view through the optical system. The two mirrors are rotationally symmetric about the optical axis. The observer looks at the secondary mirror through a hole at the center of the primary mirror.
This telescope design places a small secondary mirror far from the eye of the observer, and the observer has to look through several refractive lenses to see the image generated by the small and far away secondary mirror. The optical quality and the magnification is largely determined by the mirrors, but to allow a view of the image several refractive lenses are needed. The secondary mirror conceals objects in the center of the field of view, because it is at the center of the field of view of the primary mirror. Lenses cause chromatic aberration and also limit the range of electromagnetic radiation that the telescope may use.
Several telescopes using three or more mirrors and no lenses are disclosed in U.S. Pat. Nos. 3,674,334; 4,101,195; 4,215,273; 4,226,501; 4,240,707; 4,265,510; 4,632,521; 4,645,314; 4,733,955; 4,737,021; 4,804,258; 4,834,517; and 4,964,706. These systems project real images directly on sensors. Some of these systems are also off-axis and eliminate the central obstruction by the secondary mirror present in prior telescopes. These systems use more mirrors than necessary, resulting in increased cost and optical aberration.
A combination of two reflecting and two refracting surfaces which are rotationally symmetric about the optical axis, is used in the Maksutov optical systems, as described in "New Catadioptric Meniscus System" by D.D. Maksutov, Journal of Optical Society of America, Vol. 34, No. 5, May, 1944. Maksutov developed the technique whereby several refractive surfaces, e.g. lenses, are used to correct optical aberrations caused by two spherical mirrors. The Maksutov microscope is typical of such a system. Recent off-axis designs based upon the Maksutov approach are shown in U.S. Pat. Nos. 4,196,961; 4,293,186; 4,344,676; 4,711,535; 4,747,678 4,812,028; and 4,964,705. Geometrical and chromatic aberrations are still major problems in such systems.
U.S. Pat. No. 4,927,256 discloses a two curved mirror, side-looking telescope wherein the primary mirror is an off-axis segment of a concave paraboloid surface and the secondary mirror is a concave segment of an hyperboloid. The secondary mirror is moved off-axis. Because of the specific shape selected, this optical system will not be suitable for direct forward viewing by the eye and there is an increased optical aberration in the periphery. Additional optical elements will be required to overcome those limitations, as admitted by the authors.
U.S. Pat. No. 4,812,030 discloses a zoom system built around a rotationally symmetric co-axial two-mirror system. This system suffers from the central concealment effect resulting from placement of the secondary mirror at the center of the field of view of the primary mirror mentioned before.
Conventional rearview mirrors in cars have many blind spots. Rearview periscope mirror systems for use with automobiles and other vehicles attempting to eliminate such blind spots heretofore have employed planar mirrors. Some have required a special opening in the roof of the vehicle for communication between interior and exterior mirrors. With planar mirrors, the internal mirrors are small and result in limited field of view. Also, the external mirror needs to be large because of linear divergence of the line of sight from the vehicle driver. Typical of these are the periscope structures disclosed in U.S. Pat. Nos. 3,058,395; 3,704,062; 3,909,117; 3,914,028; 3,915,562; 3,947,096; 3,979,158; 4,114,989; 4,120,566; and 4,277,142.
Prior attempts also have been made to provide a rearview periscope composed of only two mirrors. Exemplary of these are the systems disclosed in U.S. Pat. Nos. 4,110,012 which employs one elliptically curved mirror and an associated flat mirror; and 4,033,678 which utilizes two cylindrically concave mirrors. In both of these systems the use of two mirrors to obtain an upright image of objects to the rear resulted in a very short distance of in-focus viewing sandwiched between relatively long distances of out-of-focus viewing.